yes, the number of detected stars jumped out as at me as well – try turning up the noise reduction or the minimum star width in the alignment settings to bring this number down. The alignment actually only uses the brightest 15 stars or so from the stack and from the new frame, but given that massive number of stars being detected I guess it's possible that the brightest 15 from the 2 lists may not have much overlap.

Update..... I lowered the maximum star value to 12 and raised the minimum star value to 2. This puts a tighter limits on the stars it will accept. I only had to do this on really long exposures because the stars get blown out badly. This lowered the amount of stars it was finding and it seemed to stack perfectly.

I thought about raising the black level from 50% to 60% or higher. By doing this would it cover up dim or small stars? Is this a better option to raising the minimum star value?